arXiv subjects

Jose J. Fonseca

Publications and source records attributed to Jose J. Fonseca.

3 recordsLinked to original sources

arXiv2025

Nitrogen-Vacancy Magnetometry of Edge Magnetism in WS2 Flakes

Two-dimensional (2D) magnets are of significant interest both as a platform for exploring novel fundamental physics and for their potential in spintronic and optoelectronic devices. Recent bulk magnetometry studies have indicated a weak ferromagnetic response in WS2, and theoretical predictions suggest edge-localized magnetization in flakes with partial hydrogenation. Here, we use room-temperature wide-field quantum diamond magnetometry to image pristine and Fe-implanted WS2 flakes of varying thicknesses (45-160 nm), exfoliated from bulk crystals and transferred to NV-doped diamond substrates. We observe direct evidence of edge-localized stray magnetic fields, which scale linearly with applied external magnetic field (4.4-220 mT), reaching up to 4.7 uT. The edge signal shows a limited dependence on the flake thickness, consistent with dipolar field decay and sensing geometry. Magnetic simulations using five alternative models favor the presence of edge magnetization aligned along an axis slightly tilted from the normal to the WS2 flake plane, consistent with spin canting in antiferromagnetically coupled edge states. Our findings establish WS2 as a promising platform for edge-controlled 2D spintronics.

cond-mat.mes-hall↗

arXiv2020

Unexpected Near-Infrared to Visible Non-linear Optical Properties from Two-Dimensional Polar Metals

Near-infrared-to-visible second harmonic generation from air-stable two-dimensional polar gallium and indium metals is described. The photonic properties of 2D metals - including the largest second-order susceptibilities reported for metals (approaching 10nm$^2$/V) - are determined by the atomic-level structure and bonding of two-to-three-atom-thick crystalline films. The bond character evolved from covalent to metallic over a few atomic layers, changing the out-of-plane metal-metal bond distances by approximately ten percent (0.2 $\unicode{x212B}$), resulting in symmetry breaking and an axial electrostatic dipole that mediated the large nonlinear response. Two different orientations of the crystalline metal atoms, corresponding to lateral displacements < 2 $\unicode{x212B}$, persisted in separate micron-scale terraces to generate distinct harmonic polarizations. This strong atomic-level structure-property interplay suggests metal photonic properties can be controlled with atomic precision.

cond-mat.mtrl-sci↗

arXiv2019

Enabling remote quantum emission in 2D semiconductors via porous metallic networks

The interaction between two-dimensional crystals (2DCs) and metals is ubiquitous in 2D material research. Here we report how 2DC overlayers influence the recrystallization of relatively thick metal films and the subsequent synergetic benefits this provides for coupling surface plasmon-polaritons (SPPs) to photon emission in 2D semiconductors. We show that annealing 2DC/Au films on SiO2 results in a 'reverse epitaxial' process where initially nanocrystalline Au films become highly textured and in close crystallographic registry to the 2D crystal overlayer. With continued annealing, the metal underlayer dewets to form an oriented pore enabled network (OPEN) film in which the 2DC overlayer remains suspended above or coats the inside of the metal pores. This OPEN film geometry supports SPPs launched by either direct laser excitation or by light emitted from the TMD semiconductor itself, where energy in-coupling and out-coupling occurs at the metal pore sites such that dielectric spacers between the metal and 2DC layer are unnecessary. At low temperatures a high density of single-photon emitters (SPEs) is present across an OPEN-WSe2 film, and we demonstrate non-local excitation of SPEs at a distance of 17 μm with minimal loss of photon purity. Our results suggest the OPEN film geometry is a versatile platform that could facilitate the use of layered materials in quantum optics systems.

cond-mat.mes-hall↗